The present invention relates to materials and methods for the preparation of high molecular weight acrylic based water-soluble polymers, and in particular to acrylamide based polymers for use as flocculants, dewatering and thickening agents.
Synthetic polymeric materials such as polyacrylamides have long been used as flocculants in wastewater treatment and other processes in which it is desirable to remove suspended solid materials. U.S. Pat. No. 3,235,490 discloses that certain crosslinked acrylamide based polymers are effective flocculants due to an electrostatic attraction followed by a sweeping action of a xe2x80x9cfilamentaryxe2x80x9d network provided by the crosslinked polymer. It is also disclosed that the formation of non-homogeneous products, which have a tendency to precipitate from solution, contributes to the effectiveness of the crosslinked polymers.
U.S. Pat. No. 4,720,346 describes a process for producing partially crosslinked acrylamide based cationic and anionic flocculants for use in municipal and industrial wastewater treatment. The crosslinked polymeric material is said to have controlled non-linearity which is caused by crosslinking preformed polymers, or by polymerizing acrylamide monomers in the presence of a crosslinking agent. However, as disclosed therein, the intrinsic viscosity of the water soluble polymers decreases as the concentration of crosslinking agent is increased. Vigorous shearing action is required prior to or during flocculation to make the crosslinked or particulate polymeric material soluble and to increase the intrinsic viscosity. A high dosage of the polymeric material is required for effective removal of solids.
U.S. Pat. No. 4,759,856 describes a flocculation process for removing suspended solids in an aqueous suspension. A polymeric material comprising crosslinked water swellable polymer is added to water to form an aqueous composition, which is in turn added to an aqueous suspension for flocculation. The polymeric material must be subjected to shear during flocculation or in the aqueous composition before adding to the suspension.
Shearing or vigorous stirring of polymeric materials prior to or during flocculation requires process equipment, which is often not available at the wastewater treatment site. The additional shearing and stirring steps inevitably complicate the flocculation process and reduce the cost efficiency.
This invention provides a high molecular weight water-soluble polymer which exhibits enhanced flocculating or thickening performance in the absence of high shear. This high molecular weight water-soluble polymer is obtained by polymerizing one or more polymerizable monoethylenically unsaturated monomers. The monoethylenically unsaturated monomers in the reaction mixture are polymerized in the presence of a chain branching agent. Specifically, the chain branching agent is added to the reaction mixture continuously or semi-continuously, preferably until the polymerization reaction is substantially completed. The polymerization reaction can be conducted in inverse-emulsion, solution, solution-gel, or by a precipitation polymerization process.
Typically, the chain branching agent is a water soluble multifunctional monomer having at least two unsaturated groups, and is added at a total amount of from about 1 ppm to about 5xc3x97104 ppm based on the total amount of the reaction mixture. In a preferred embodiment, N,N-methylene-bis-acrylamide, also referred to as MBA, is used as the chain branching agent at a total amount of from about 1 to about 1000 ppm, preferably from about 2 to about 300 ppm, more preferably from about 4 to about 100 ppm, and most preferably from about 4 to about 25 ppm based on total monomers. The methylene-bis-acrylamide is added continuously or semi-continuously during the polymerization reaction until a total monomer conversion of about 90% is achieved.
Suitable monomers for use in the process of this invention include, but are not limited to, acrylamide, methacrylamide, N-vinyl methyl acetamide, N-vinyl methyl formamide, dialkylaminoalkylmethacrylamide, sulphomethylated acrylamide, vinyl acetate, vinyl pyrrolidone, methacrylic esters, styrene, acrylonitrile, methacrylic acid, itaconic acid, acrylamido methyl propane sulphonic acid, allylsulphonate, sodium vinyl sulphonate, sodium acrylate, diallyldimethylammonium chloride, methacrylamidopropyl trimethylammonium chloride, dialkylaminoalkyl methacrylate, dialkylaminoalkyl acrylate, dialkylaminoalkyl acrylate methyl chloride, and quaternaries or acid salts thereof. At least one of the above monomers is used. Two or more of the above monomers may be copolymerized.
In one aspect of the invention, the polymer of this invention is polyacrylamide, i.e., an acrylamide based high molecular weight water-soluble polymeric material. At least one acrylamide based monomer is normally incorporated into the polymer. For example, a preferred cationic monomer reaction mixture may contain acrylamide and dialkylaminoalkylacrylate (as an acid salt or quaternary salt) in a concentration of at least 30 wt % of the cationic monomer. A preferred anionic monomer system can include acrylamide and methacrylic acid as comonomers and acrylic acid as monomer.
In another aspect of this invention polyacrylamide is used as a flocculant for treating an aqueous suspension such as residential or industrial wastewater. Accordingly, a process for flocculating an aqueous suspension of suspended solids is provided comprising adding to the suspension a flocculating amount of the polymer of this invention, such that an aqueous medium containing flocculated suspended solids is formed. The flocculated suspended solids are then separated from the aqueous medium in a conventional dewatering step, e.g., centrifuge, piston press, or belt press dewatering. Typically the flocculating amount, i.e., the effective dosage, of the polyacrylamide flocculant material of this invention ranges from about 1 to about 105 ppm. The optimal dosage can vary, depending on the solid suspension to be flocculated and the properties of a particular polyacrylamide used. Preferably from about 10 to about 1000 ppm of the polyacrylamide of this invention is used.
While not wishing to be bound by any theory, it is believed that in the polymerization process of this invention, regular branching and/or partial crosslinking along the polymer backbone is achieved as a result of the introduction of the chain branching agent into the reaction mixture in a constant manner or stepwise as a function of time. The polymeric material has increased homogeneity in its chemical structure and network and has sufficiently high intrinsic viscosity. As a result, the polymeric material, exhibits enhanced flocculating performance without being subjected to shearing action.
In addition, it is also believed that, as compared to randomly crosslinked or partially crosslinked polymeric flocculants known in the art, more attachment points are available in the polymer of this invention for attaching onto the solids in the aqueous suspension due to the regularly spaced branching and/or regular partial crosslinking. The polymeric material of this invention is a more facile and efficient flocculant. A significantly lower dosage of the polymeric material of this invention can be employed in a flocculating process while shear action or vigorous stirring either before or during the flocculating process can be avoided. Therefore, the high molecular weight polymer of this invention can exhibit superior sludge dewatering characteristics and improved performance as thickening agents.
As compared to comparable branched polyacrylamide materials prepared by conventional methods, the polyacrylamide material according to this invention exhibits a significantly higher intrinsic viscosity, a lower level of aggregation/association, and a significantly higher weight-average sedimentation coefficient. Typically, the water soluble acrylamide based polymeric flocculant material according to this invention has an intrinsic viscosity (1 M NaCl at 25xc2x0 C.) of greater than about 2 dl/g, and preferably greater than about 2.5 dl/g. The water soluble acrylamide based polymeric flocculant material of this invention can also exhibit a sedimentation coefficient of the soluble portion of greater than about 5 Svedberg, preferably greater than about 10 Svedberg (measured in 0.05 M NaCl), and an association/aggregation to chain branching agent ratio of less than about 3, preferably less than about 2.5 (measured in 0.005 M NaCl at 25xc2x0 C.).
Accordingly, this invention provides a water soluble polymer which has regularly spaced branches and/or crosslinks along the polymer backbone. The polymer prepared according to this invention exhibits increased intrinsic viscosity, higher weight-average sedimentation coefficient, and lower aggregation/association than branched flocculating polymers formed from comparable compositions. The polyacrylamide of this invention is especially effective in flocculating suspended solids without shearing.
The foregoing and other advantages and features of the invention, and the manner in which the same are accomplished, will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunction with the accompanying examples, which illustrate preferred and exemplary embodiments.